Sometimes in subterranean wells it is necessary to plug small voids generally to prevent unwanted fluid flow. These voids may be cracks in the cement, or micro-annuli between the casing and the cement or between the cement and the formation. These voids could also be pore spaces between gravel in gravel-packs in zones that are producing too much water to remain economic. A common method of plugging these voids is to inject a cement formulation where the cement is made from micro-fine cement particles (see p 247 Well Cementing, 2nd Edition, E. Nelson and D. Guillot editors, Schlumberger 2006) where the micro-fine particles have a median size of 3-5 microns. The process is generically called squeeze cement (pp 521-543 Well Cementing, 2nd Edition, E. Nelson and D. Guillot editors). U.S. Pat. No. 6,312,515 also discloses a squeeze cementing method using a cement formulation with micro-fine particles. However, even with this technology there is a lower limit to the size of the voids that can be filled when using micro-fine cementing technology because of the particle size.
It is also known to use calcium carbonate in a silicate containing solution in sealant compositions. U.S. Pat. No. 4,762,443 describes a fluid composition comprising a sodium metasilicate and a calcium carbonate which is used to make porous material impervious by injecting the composition into the material, which will then harden and set over time.
However the voids (microannuli, fractures and pores) in subterranean wells that the sealant composition needs to penetrate can be very small and narrow, and therefore the solid particles in sealant compositions need to be small enough to effectively penetrate into these gaps. However when the two components are mixed to form the composition for injecting, the reaction between the calcium carbonate and sodium metasilicate can occur very quickly. This can cause problems due to the onset of thickening of the composition before it has sufficiently penetrated the pores and fractures of the formation. If the composition becomes too thick to pump properly the sealant composition may not be correctly placed in all the voids before it sets. This may reduce the extent of, or even prevent the formation of, the barrier that is meant to be created throughout the formation. In addition, the subterranean formations may have temperatures higher than surface conditions which lead to even higher reaction rates between the calcium carbonate and the silicate solution.
Therefore it is an object of the invention to provide a sealant composition that can penetrate into the narrow voids, such as fractures and pore spaces, of a wellbore.